1. Field of the Invention
The present invention concerns a method to reduce a deviation from a predetermined desired pitch (spacing) that occurs in a print direction between two points, of the type wherein the at least one print image is generated in a printing step with at least one print head of a printing device on a substrate given a relative movement between the print heads and the substrate; wherein an initial deviation from a desired pitch between the two points is determined in a determination step preceding the print step; and correction information to reduce the deviation is determined from the determined initial deviation; and control signals for the at least one print head are generated dependent on the correction information in the printing step to generate the at least one print image. Moreover, the application concerns a corresponding printing device.
2. Description of the Prior Art
In franking machines, as well as in other printing devices in which a substrate is printed in a single movement, the problem frequently exists that the print image to be generated has a dimension transversal to the printing direction that is larger than the print width provided by the employed print head type. Therefore, it is necessary to use multiple print heads in order to achieve the required print width. Since the housing of the employed print heads is normally wider than the actual area used for printing, it is typically not possible to arrange the printing regions of the print heads (in the printing direction) at the same level since in this case flush connection of the partial images (transversal to the printing direction) is not possible; consequently, no gapless print image can be generated. Therefore, it is necessary to arrange the employed print heads offset from one another in the printing direction and transversal to the printing direction in order to achieve a flush connection (possibly even a slight overlap) of the partial images.
This design has the result that pixels that lie next to one another on the print image but are generated by different print heads must in part be printed with a distinct time interval. For example, if a first pixel is printed at the edge of the first partial image by the first thermotransfer print head at a first point in time, the second pixel lying directly adjacent to the first pixel at the edge of the second partial image is only printed by the second print head when the substrate (for example a letter that is transported by a corresponding transport device) has overcome the distance in the printing direction between the two regions of the two print heads that are used for printing.
The relative position between the print heads and the substrate, and therefore reaching the position or, respectively, the point in time at which the second pixel is to be printed, is typically registered via a corresponding measurement device. This is typically an encoder connected with the drive of the transport device that provides at its output a definite number of measurement signals in the form of encoder pulses per unit distance of the relative movement (between the substrate and the print heads) that is traveled. Given such transport devices, rotating elements (in particular rollers and similar elements) are typically used in order to transport the substrate to be printed.
Due to the illustrated time offset of the printing of adjacent points by different print heads, the imprints of such printing devices in the print direction can exhibit an offset between the two adjoining print images (partial images) as a deviation form a predetermined desired pitch between points of the (total) print image. It is hereby understood that in this case the desired pitch (in the printing direction) between the immediately adjacent points of the two (partial) print images is equal to zero; thus no offset of the two (partial) print images is desired at all. This offset typically has a static offset portion and a periodic offset portion. Such an offset between the two adjoining (partial) print images in the field of generation of franking images is not least due to the increasing requirements for machine readability of such print images.
The static offset portion is typically due to the diameter error of the drive elements, or is based on position errors due to tolerances in the installation of the print heads. For example a deviation of the diameter of the transport roller for the substrate from its desired value is due to the fact that reaching the print position or, respectively, the printing point in time too early (smaller diameter) or too late (larger diameter) is registered in the evaluation of the measurement signals (for example counting the encoder pulses) of the shaft encoder (encoder) connected with the transport roller.
This problem is typically solved in such known printing devices via readjustment of the print heads. Typically, a test pattern (for example a nonius [vernier] pattern) is printed out by the two print heads and the separation of the two regions used for printing (thus the separation of the two nozzle rows given the use if inkjet print heads) in the printing direction is determined using the position of the minimal offset and is passed as correction information to the controller of the print heads. However, only static offset proportions that are an integer multiple of the print resolution can be entirely corrected with this. For example, if a print resolution of 300 dpi is provided, the maximum remaining residual error is still ± 1/600 in or, respectively, approximately ±42 μm. A normal remaining residual error that can significantly impair the quality of the imprint cannot be corrected by this.
Furthermore, the occurring periodic offset portion is due to variable interferences in the printing with different print heads. Since adjacent points are printed by different print heads at different points in time, under the circumstances an interference present upon printing the first point with the first print head has already-subsided again when the immediately adjacent second pixel is printed with the second print head.
There are multiple causes for this periodic offset proportion, such as an eccentric connection of the encoder, an eccentricity of the driving rollers (same period duration but deviating phase position), ovality errors of the driving rollers (deviating period duration and deviating phase position) as well as shocks that can occur due to changes of the engagement ratios of the drive elements.
The causes just described for the static and period offset portions do not, however, occur only given the use of multiple print heads arranged with offset. Rather, they also have an effect in printing devices with a single print head. Here they affect the separation of following points in the print direction and draw attention as constant (static portion) or periodically variable (periodic portion) expansion and/or compression of the print image in the print direction.
A method and a device for calibration of driver signals of a print head is known from the disclosure document DE 10 2004 053 146 A1. The calibration is implemented after exchanging a cartridge. Four parameters of the driver signal are calibrated: the duration of the main drive pulse; the duration of the preheating pulse; the time interval between pre- and main drive pulse; and the driver voltage. For each of these parameters, multiple test prints are printed depending on different respective values of a parameter. The respective parameter value that leads to the best print result is subsequently selected. Neither a dynamic observation of the printing device nor a detection of a periodic offset proportion occur.